I sounds like one of the classic cases of cheating from the description of how he's changed. Has he upped his grooming or started getting new clothes or cologne without you mentioning anything of it?

Also, it could go the other way. A lot of times, when people are cheating theyre the total opposite. It gives them a bigger labido and makes them happier because of this new person so instead of being stand off-ish and irritated they have more energy etc...

Also, is he super protective of things like his cell phone? If he's doing something he doesnt want you to know about chances are there will be phone calls or texts to support it so if he flips out when you touch his phone that could be an indication.

Ive found that everyone worries every know and then but theres a difference between that and that feeling in the pit of your stomach where you just know something is up. Ive noticed that my worrying is usually wrong but my feeling in the pit of my stomach has always been right.

And it might not be cheating, he might just be not into it anymore. Maybe the sex has become routine and boring after a year and a half. Have you tried anything to spice it up bit to get him in the mood? This is easier for me then you because my boyfriend and I don't live together but one of his biggest fantasies is his girlfriend coming over to his house in underwear and a bath robe and no one had ever done that for him before. So when I noticed that things started to get a little routine I did it for him and hes been all over me since.

Even the littlest things like canging your hair style or switching up an outfit can make a guy go crazy for you. So my advice to you is to try something like having something set up for him when he gets home... candles y'know etc... and if that doesnt do the trick of spicing it up then you might have a problem.